2 x-axis labels below each other for bar diagram

Hi there!

I want to make a bar plot of 8 sets of 3 bars. And, I want to label the sets of 3 which represents the wave height (e.g.H=1) and the 3 bars the different periods (T=3, T=4, T=5). All sets have different wave heights values, all columns have different period values. So when I look at the bar plot, I immediately want to see which bar responds to which wave height and period.

My idea was to show the periods on the x-axis and below that axis the wave heights. But I can not find how to do that...

Can some one give me hint??

Thnx in Advance!

[dpb]

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